In electrolytic processes, it has been found that the hydrogen discharge overpotential at the cathode of electrolysis cells is reduced by a coating comprising an alloy of nickel and zinc. In order to activate such coating, the alloy coated electrodes are immersed in a leach solution, commonly sodium hydroxide of moderate concentration, which removes a portion of the zinc from the alloy and leaves a nickel surface of high porosity and surface area. Patents describing this coating and leaching process include Hahndorff, U.S. Pat. No. 3,272,728, Canadian Pat. No. 955,645, and Japanese Patent Application Publication No. 6611, published Aug. 7, 1956.
U.S. Pat. No. 4,104,133 describes a process by which diaphragm cell cathode cans are coated with a nickel-zinc alloy from an electroplating solution of nickel and zinc. Anode material used in the plating process may be an alloy of nickel and zinc, an insoluble anode or, as is most common, only nickel anodes are used. The use of only nickel anodes allows for easier handling of the anode and cathode assembly, particularly when the anodes are positioned within the cathode can prior to cleaning, and the caustic cleaning solution could result in dissolution of large amounts of zinc from a combined nickel/zinc anode.
Following the plating procedure, the plated cathode can having a nickel-zinc alloy coating thereon is transferred to and immersed in a leach tank containing a solution of approximately 10 percent sodium hydroxide. Over a period of time, most of the zinc in the nickel-zinc deposit leaches out of the deposit and into solution in the leach system. It can be seen that within a short period of time following the leaching of a number of cells, a large concentration of zinc is built-up within the leach solution. In a production plating operation, as many as 20 cells per day are plated and subsequently leached producing approximately 1000 pounds of zinc in the leach solution. Such a large amount of zinc presents a critical pollution control problem as well as representing a considerable expense in metal. A process which removes zinc from the leach solution and also recovers the zinc in a form suitable for reuse within the electroplating cycle would be extremely advantageous.
It is therefore a principal object of this invention to remove leached zinc ions from the leach solution thereby avoiding the pollution problems and costs associated therewith.
It is another object of this invention to utilize recovered zinc in a process which restores the zinc concentration in the electroplating bath for the deposition of a nickel-zinc alloy onto electrolysis cell cathode cans.
Further objects and advantages of the invention will become apparent to those skilled in the art upon the reading and understanding of this specification.